The present invention relates to a process for preparing neopentyl glycol by hydrogenating hydroxypivalaldehyde in the liquid phase over a nickel-containing catalyst in the presence of more than 15% by weight of water, based on the starting mixture.
Polyhydric alcohols or polyols possess considerable economic significance as a condensation component for forming polyesters or polyurethanes, synthetic resin coatings, lubricants and plasticizers. In this context, polyhydric alcohols of interest are particularly those which are obtained by a mixed aldol addition of formaldehyde with iso- or n-butyraldehyde. The aldol addition between formaldehyde and the appropriate butyraldehyde first forms an aldehydic intermediate which then has to be reduced to the polyhydric alcohol. An industrially important example of such a polyhydric alcohol obtainable by this process is neopentyl glycol [NPG, 2,2-dimethylpropane-1,3-diol], which is obtained by mixed aldolization of formaldehyde and isobutyraldehyde. The aldol addition reaction is performed with equimolar amounts in the presence of basic catalysts, for example, alkali metal hydroxides or aliphatic amines, and first affords the isolable hydroxypivalaldehyde (HPA) intermediate. This intermediate can subsequently be converted with excess formaldehyde in accordance with the Cannizzaro reaction to neopentyl glycol to form one equivalent of a formate salt. In this configuration of the reduction step, the formate salt is therefore obtained as a coproduct. However, also implemented industrially is the catalytic hydrogenation of hydroxypivalaldehyde in the gas and liquid phase over a metal catalyst. Suitable hydrogenation catalysts have been found, according to EP 0 278 106 A1 to be nickel catalysts which may comprise further active metals, such as chromium or copper, and additionally activators. The crude aldolization mixture is subsequently catalytically hydrogenated without preceding separation into its constituents or removal of individual components. Since formaldehyde is typically used as an aqueous solution, for example as a 37% by weight solution, water is present in the aldolization mixture to be hydrogenated. The crude hydrogenation product obtained can then be worked up by distillation according to the teaching of EP 0 278 106 A1.
A further process for hydrogenation of hydroxypivalaldehyde to neopentyl glycol in the liquid phase in the presence of nickel catalysts is known from WO 99/035112 A1. Particular reference is made to the harmful influence of too high an amount of water on the stability of the nickel catalyst during the hydrogenation process. There are reports of catalyst damage and also of a selectivity decline at the expense of neopentyl glycol as a result of the presence of water. WO 99/035112 A1 therefore proposes limiting the amount of water to less than 15% by weight in the hydrogenation of hydroxypivalaldehyde to neopentyl glycol. The hydrogenation temperature of 100° C. should also not be exceeded in the known processes, since the employment of higher hydrogenation temperatures in the presence of nickel catalysts results in enhanced by-product formation, such as the formation of neopentyl glycol monoisobutyrate or neopentyl glycol monohydroxypivalate.
WO 98/17614 A1 also considers the hydrogenation of hydroxy-pivalaldehyde to neopentyl glycol by the liquid phase process in the presence of nickel catalyst. In the known process, isobutyraldehyde is first reacted with an aqueous formaldehyde solution in the presence of a tertiary alkylamine to give a crude mixture comprising hydroxypivalaldehyde, which subsequently is subjected to an extraction with an aliphatic alcohol. The low-boiling components are distilled out of the organic phase, and the higher-boiling components which comprise hydroxypivalaldehyde are hydrogenated. For workup, the hydrogenation product is extracted with water, which transfers neopentyl glycol to the aqueous phase. Neopentyl glycol is then isolated from the aqueous phase by distillation. Extraction and distillation steps connected upstream of the hydrogenation stage reduce the amount of the water present in the hydrogenation stage. In the known processes, the hydrogenation stage should be performed within a temperature range from 120° C. to 180° C.
According to U.S. Pat. No. 6,268,539 B1 the aldolization product obtained from the reaction of isobutyraldehyde and an aqueous formaldehyde solution under triethylamine catalysis is first distilled. The resulting water-containing distillation residue is subsequently hydrogenated at 70 to 120° C. in the presence of Raney nickel which comprises molybdenum as a promoter. The known liquid phase process is characterized by the use of a specific self-aspirator agitator which ensures intensive mixing between the liquid and gaseous phases. As a result of this specific reactor configuration, only low hydrogenation pressures in the range from 0.55 to 12.4 MPa are required.
The reaction regime known from EP 0 395 681 B1 also allows the liquid phase hydrogenation of hydroxypivalaldehyde to be performed in the presence of Raney nickel using a specific reactor design in which hydrogen gas is passed intensively through the liquid reaction mixture. This stripping effect removes traces of the tertiary amine used as an aldolization catalyst and compounds thereof which promote the decomposition of the hydroxypivalaldehyde in the hydrogenation stage. According to the teaching of EP 0 395 681 B1, there is no need to employ high pressure. The crude mixture used in the hydrogenation stage contains 10 to 35% by weight of water.
For the liquid phase hydrogenation of hydroxypivalaldehyde to neopentyl glycol in the presence of nickel catalysts, either a special reactor design is required, or only low water contents in the crude hydroxypivalaldehyde are permitted for the use for hydrogenation, in order to convert hydroxypivalaldehyde at high conversion with high selectivity to neopentyl glycol. In some cases, the crude hydroxypivalaldehyde must first be subjected to an additional extraction and distillation in order to reduce the water content in the product to be hydrogenated.
It is, however, desirable to hydrogenate the reaction product from the alkylamine-catalyzed aldol addition of isobutyraldehyde with an aqueous formaldehyde solution directly and without purification steps in the presence of a common, industrially available nickel catalyst in the liquid phase.
It is therefore an object of the invention to develop a process which is simple to perform in technical terms and enables, with economically acceptable means, neopentyl glycol to be obtained by alkylamine-catalyzed aldol addition.